Magnetic recording media are widely used as audio tapes, video tapes and floppy disks. A magnetic recording medium is basically comprised of a non-magnetic support having thereon a magnetic layer containing a binder and ferromagnetic particles dispersed therein.
It is necessary for a magnetic recording medium to have high levels of various characteristics such as electromagnetic conversion characteristics, running durability and running efficiencies. With the recent development of 8 mm video tape recorders, it is particularly necessary for a video tape to produce a high video output, have excellent capabilities for reproducing original images, and in particular have excellent electromagnetic conversion characteristics.
There have been various approaches for improving the electromagnetic characteristics of a magnetic recording medium of these, the approach of improving the characteristics of ferromagnetic particles as a magnetic recording substance is direct and effective. Accordingly, innovators continually attempt to achieve high density recording by ever more finely granulating ferromagnetic particles.
Another approach is to use iron oxide modified with different metals such as cobalt instead of using iron oxide per se as the material for forming the ferromagnetic particle. Recently, ferromagnetic metals such as iron, nickel or cobalt and alloys containing the above metals have been used, for this purpose.
A magnetic recording medium having essentially good electromagnetic conversion characteristics can now be obtained by using the thus improved ferromagnetic particles. However, it is difficult to produce a magnetic recording medium which achieves improved electromagnetic conversion characteristics by virtue of improving the ferromagnetic particles. This is because the dispersibility of the ferromagnetic particles decrease as the particles become more finely granulated, and dispersibilities of .gamma.-iron oxide, cobalt-coated .gamma.-iron oxide and ferromagnetic metal particles in this order tend to become reduced, which is the characteristics of ferro-magnetic particles.
To overcome this difficulty in preparing a magnetic coating composition, the steps of mixing, kneading and dispersing have been carried out for a long period of time in an attempt to improve the dispersibility of the ferromagnetic particles. However, in many cases the beneficial characteristics of the ferromagnetic particles are deteriorated by the shearing force which occurs upon mixing, kneading and dispersing. Furthermore, the long period of time necessary for producing such a magnetic recording medium is a problem in view of operation efficiencies.
In this connection, methods of effectively dispersing ferromagnetic particles without substantially modifying the conventional method for preparing a magnetic recording medium have been investigated and suggested. One method comprises using ferromagnetic particles which have been surface treated with a surface processing agent such as a silane coupling agent. Another method comprises pre-treating ferromagnetic particles with a component (dispersing agent) such as a fatty acid which improves the dispersibility of the ferromagnetic particles as disclosed in JP-B-54-7074 and GB-A-1450241 (The term "JP-B" as used herein means an examined Japanese patent publication). Still another method comprises adding the above-described fatty acid during preparation of the magnetic coating composition.
However, it was confirmed by the present inventors that even the above methods could not sufficiently improve the dispersibility of ferromagnetic particles.
That is, when ferromagnetic particles are surface-treated with the above-described silane coupling agent, the state of dispersibility thereof in a magnetic coating composition is generally stable, but in some cases, the compatibility toward the resin components actually deteriorates. Accordingly, this method cannot sufficiently improve the state of dispersibility of the ferromagnetic particles in a magnetic layer. Further, the fatty acids generally added as a lubricating agent in a magnetic layer have a dispersing effect upon ferromagnetic particles. Such a dispersing effect cannot fully be obtained unless one uses a larger amount of the fatty acids which function as dispersing agents than the amount of the fatty acids which function lubricating agents in a magnetic layer. However, when a large amount of fatty acids is used, another problem arises: the binder becomes plasticized.
Furthermore, as the dispersibility of ferromagnetic particles is improved by the above method, the surface smoothness of the magnetic layer is hightened. Therefore, the friction coefficient on the surface of the magnetic layer increases, thereby deteriorating the running properties and running durability.
JP-A-63-42025 discloses a magnetic recording medium using benzoic acid and the salt thereof as a dispersing agent, which medium can avoid the problem of the binder becoming plastisized when the above-described fatty acids are used (The term "JP-A" as used herein means a published unexamined Japanese patent application). This magnetic recording medium has good dispersibility and prevents the binder from plasticizing, but cannot have a satisfactory running property and running durability.